A thyristor, that is, a PNPN device with a gate terminal, can be used to protect an electrical transmission system from transient voltages. A thyristor connected in shunt with a signal line of a telephone system, for example, has a low-current high-impedance state during normal conditions and switches to a high current low impedance state should the conditions change to cause the line voltage to go below the gate voltage of the device, the thyristor being so connected that its outer P-type region is positive with respect to its outer N-type region and its gate electrode is not positive relative to its outer N-type region.
A thyristor connected in shunt with a signal line recovers to its low-current high impedance state following the occurrence of a voltage on the signal line causing its conduction because the thyristor requires a current above a set level to keep it in its low impedance state after it has been switched on. That current is the holding current for the thyristor and must exceed the current which can be obtained through the signal line, in order that the thyristor returns to its high-impedance state following the occurrence of a transient voltage which switches it on.
A thyristor can be designed to have a high holding current, but such a thyristor requires a high gate current and, in a system protected by the device, the high gate current which the device draws can charge the system capacitance to a voltage high enough to provide a voltage damaging to many components.